1. Technical Field
The present invention relates to a liquid crystal device, a fabricating method, and a fabricating apparatus thereof. More particularly, it relates to a method for fabricating an electrically-controlled birefringence effects-type vertically aligned liquid crystal device.
2. Prior Art
Electrically-controlled birefringence effects (hereinafter referred to as ECB)-type vertically aligned liquid crystal devices change wavelength dispersion characteristics, quantity of transmitted light, and color of transmitted light by changing the birefringence rate of nematic liquid crystal wherein its molecular orientation is changed in an electric field. Generally, in the ECB-type vertically aligned liquid crystal devices, transparent electrodes are formed on glass substrates by the evaporation or sputtering method, and the like. Alignment films for vertical alignment are deposited on the transparent electrodes, and the above two glass substrates are assembled so that the alignment films may face each other to fill a liquid crystal material having a negative dielectric anisotropy between the glass substrates. Further, polarizers or polarizer films are mounted on the outside of the glass substrates.
As shown in FIG. 8(a), when no voltage is applied, liquid crystal molecules 12 respectively have a pretilt angle of about 86 to 89 degrees. A pretilt angle means an angle which tilts to a substrate. In FIGS. 8(a) and 8(b), substrates and electrodes are omitted. Further, when the pretilt angle is previously adjusted to be 90 degrees (perpendicular to the substrate), it is not fixed to which direction the liquid crystal molecules 12 are tilted at the time of applying a voltage across the transparent electrodes. For this reason, it is natural to previously have a pretilt angle which is a little more oblique than 90 degrees. In this situation, incident light (Arrow mark in the drawing) is not affected by the birefringence of a liquid crystal medium, and the polarization axis of incident light is not changed, therefore, the incident light cannot pass a second polarizer 76. Because light is shielded by the second orthogonal polarizer 76 due to orthogonal arrangement of the polarization axes of the first orthogonal polarizer 74 and the second orthogonal polarizer 76. Furthermore, film-like polarizer films other than plate-like polarizers can be used for the first orthogonal polarizer 74 and the second orthogonal polarizer 76. Accordingly, when no voltage is applied, a black color is displayed.
Next, when a voltage is applied between the transparent electrodes, as shown in FIG. 8(b), the liquid crystal molecules 12 are tilted. This utilizes the property of the liquid crystal molecules each having a negative dielectric anisotropy to vertically orient in a direction of the electric field. Since the light which has launched into the liquid crystal material becomes elliptically polarized light by the effect of optical anisotropy of the liquid crystal molecules 12, light partially passes the second orthogonal polarizer 76.
There are two methods for an alignment technique to allow all liquid crystal molecules to have a uniform inclination, permitting the orientation of the liquid crystal molecules which is substantially perpendicular to a substrate when the electric field is controlled; (A) a method for forming alignment films which are vertically aligned by a rubbing treatment, (B) a method for forming alignment films by depositing a metallic compound such as silicon oxide and the like on each transparent electrode formed on a substrate. The method (A) for forming alignment films which are vertically aligned by the rubbing treatment is used for applying polyimide films for vertical alignment, and the like, which are alignment films for vertical alignment, onto the transparent electrodes formed on the substrates. Performing the rubbing treatment on the alignment films formed by the above mentioned polyimide films for vertical alignment orients all of the liquid crystal molecules in a predetermined direction. Using this method, the liquid crystal molecules may respectively have a pretilt angle (86 to 89 degrees) which is a little more inclined than a right angle with respect to the substrate.
The method (A) for forming vertically aligned alignment films by the rubbing treatment has, however, difficulties in controlling parameters at the time of rubbing treatment. The reason is that the parameters at the time of rubbing treatment are combinations of parameters experimentally found out, such as the quality of wool of the cloth, pushing depth of wool (actually the degree of a bend of wool) rotational speed of a roller, travel speed of the substrate, rubbing frequency, and rubbing pressure, or the like. Further, an addition of parameters depending on the kind of the alignment film material to the above-mentioned parameters makes parameter control difficult at the time of rubbing treatment. For example, when a streak (flaw) in the rubbing direction caused by rubbing is large, degradation of display quality may occur, more particularly, there is a possibility for the steak to be caused in a liquid crystal display. In addition, when rubbing strength (pushing depth of wool) is weak, the streak in the rubbing direction becomes shallow, and the orientation of the liquid crystal molecules are not uniform, which results in defects of liquid crystal devices.
Next, in the method (b) for forming alignment films by depositing a metallic compound such as silicon oxide on each transparent electrode formed on a substrate, a metallic oxide, such as silicon oxide, is evaporated onto the transparent electrode within a vacuum apparatus to form a column-like structure on the transparent electrode as an alignment film. This method can adjust the pretilt angle of the liquid crystal molecules to be substantially normal by making the direction of the column-like structure to be substantially vertical to the transparent electrode due to the control of the incident angle of a deposited beam with respect to the transparent electrode.
Since the method for forming alignment films by deposition has difficulties in forming a column-like structure having a uniform inclination on the transparent electrode when its deposition space is wide, its uses are limited to small sized substrates. For this reason, the substrate which has become commercially practical is 2 to 3 cm per side. Although a method for improving its uniformity by rotating a substrate while the deposition has been disclosed, but it has not become commercially practical so far. Further, due to weak alignment control, alignment abnormalities are easily caused by impurities from peripheral sealing materials, and the like.
Thus, it is difficult to fabricate ECB-type vertically aligned liquid crystal devices each having a uniform vertical alignment, so that only small-size liquid crystal devices have been partially produced so far.
The inventor of the present invention found Japanese Laid Open Patent Publication No.9-146060 in his search for prior art made in prior to the application. The patent publication discloses a method for making defective portions of liquid crystal materials inconspicuous by irradiating laser light onto the defective portions (illuminance) of the liquid crystal materials so that the liquid crystal materials each having homogeneously aligned positive dielectric anisotropy may be vertically aligned, assuming that the above-mentioned defective potions are demerit points. This method was unable to be used for fabricating a vertically aligned liquid crystal device, although the method was able to be used for repairing defective portions of homogeneously aligned liquid crystal materials.
It is an object of the present invention to provide a large-space liquid crystal device having a uniform vertical alignment, a method for fabricating it easily, and its fabricating apparatus in fabricating an ECB-type vertically aligned liquid crystal device.
The liquid crystal device according to the present invention comprises a pair of substrates, electrodes formed on each one side of the substrates, alignment films for homogeneous alignment, formed on the electrodes and facing each other by a uniform space, and a liquid crystal material including liquid crystal molecules each having a negative dielectric anisotropy, the liquid crystal material filled in a gap formed between the alignment films. The liquid crystal device may include plate-like polarizers disposed on the opposite side of each electrodes, which are plate-like polarizers or film-like polarizer films. When no voltage is applied between the electrodes, the liquid crystal molecules are vertically aligned with respect to the substrates. When an electric field is created by applying a voltage between the electrodes, the liquid crystal molecules are inclined to be tilted in a homogeneous direction with respect to the substrates because the liquid crystal molecules respectively have a negative dielectric anisotropy. This means a liquid crystal material serves as a vertically aligned liquid crystal device. Further, the above-mentioned vertically aligned liquid crystal device includes a domain-divided liquid crystal device.
The method for fabricating a liquid crystal device according to the present invention includes the steps of; forming desired electrodes on each one side of the substrates; forming alignment films for homogeneous alignment, formed on the substrates and facing each other by a uniform space; filling a liquid crystal material including liquid crystal molecules each having a negative dielectric anisotropy in a gap formed between the alignment films; and exposing the liquid crystal molecules filled between the substrates to light from either or both sides of the substrates. The liquid crystal molecules, therefore, respectively have a negative dielectric anisotropy, so that the liquid crystal molecules tend to be tilted in a direction parallel to the substrates when an electric field is created by the application of a voltage between the electrodes. Further, the disposition of half-tone masks on either or both sides of the substrates to expose the liquid crystal molecules to light from either or both sides of the substrates makes it possible to fabricate a domain-divided liquid crystal device.
The fabricating apparatus for a liquid crystal device comprises a pair of substrates, electrodes formed on each one side of the substrates, alignment films for homogeneous alignment, formed on the electrodes and facing each other by a uniform space, and a liquid crystal material including liquid crystal molecules each having a negative dielectric anisotropy filled in a gap formed between the alignment films, wherein the fabricating apparatus includes exposing means for exposing the liquid crystal material to light from either or both sides of the substrates. Exposure to light conducted by the exposing means allows the liquid crystal molecules each having a negative dielectric anisotropy which are oriented parallel to the substrates to be vertically oriented using alignment films for homogeneous alignment. In the wavelength of light emitted by the above-mentioned exposing means, a decomposition of the liquid crystal material and the alignment film occurs when the light having a wavelength of 400 nm or less is emitted. To avoid this, a filter to cut wavelengths of 400 nm or less is required for the exposing means. Further, in the light having a wavelength of 600 nm or more, a filter to cut wavelengths of 600 nm or more is required for the exposing means due to a temperature rise of the liquid crystal material. In addition, in the case a domain-divided liquid crystal device is fabricated, the fabricating apparatus includes half-tone masks each having a predetermined pattern.
The liquid crystal device according to the present invention is an ECB-type vertically aligned liquid crystal device using polyimide films for homogeneous alignment. With the use of polyimide films for homogeneous alignment, liquid crystal molecules have uniform vertical orientation. Further, the response time of the liquid crystal device according to the present invention is quick on the average compared to a conventional ECB-type vertical aligned liquid crystal device.
In accordance with the method for fabricating a liquid crystal device of the present invention, the manufacturing process other than a liquid crystal material having a negative dielectric anisotropy and rubbing is performed in a similar manner as the fabricating method of a TN-type liquid crystal device. Subsequently, liquid crystal molecules become vertically oriented by adding a process to expose the liquid crystal molecules included in the liquid crystal device to light, which enables the fabrication of an ECB-type vertically aligned liquid crystal device. The ECB-type vertically aligned liquid crystal device according to the present invention has a colored display feature, so that a process to arrange a color filter may be omitted.
The fabricating apparatus of the liquid crystal device according to the present invention includes a filter capable of cutting wavelengths of 400 nm or less and 600 nm or more, and an aligner capable of exposing the liquid crystal molecules to light having a wavelength of 400 nm to 450 nm from either or both sides of the liquid crystal device, so that the fabrication of ECB-type vertical aligned liquid crystal devices having polyimide films for homogeneous alignment has become possible.
The liquid crystal device of the present invention may be applicable to monitors for personal computers and televisions by combination with back lights. In addition, the liquid crystal device can be employed for projection systems and a head mounted display, and the like in combination with a transmission-type and a reflective-type small sized array devices.